It's there to give you an idea of the height between the plane of the neck (without the fretboard on) and the top of the soundboard. Most like there to be about 1/16" (no more though) of gap between the plane of the neck and the top of the soundboard. That little bolt tells you the gap.
This jig can tell you two things. First is if the angle at the front of the body (usually from the front of the soundhole to the front edge) is correct to the height of the bridge, and second to tell you if the fretboard extension off the neck will sit flat.
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